Conventionally, a magnetic bearing (five-axis-control magnetic bearing) which supports, of 6 degrees of freedom of a rotor (rigid body), 5 degrees of freedom using an electromagnetic force (suction force) of an electromagnet has been widely known. Such a magnetic bearing has excellent performance including a long lifetime due to a non-contact bearing portion free from abrasion, applicability to an ultra-high-speed rotary device due to an extremely small bearing loss, capability of low-vibration/low-noise rotation due to arbitrarily adjustable rigidity/attenuation properties of the bearing, and the like. Accordingly, such a magnetic bearing is used in, e.g., a vacuum pump used for a semiconductor manufacturing device or the like, a turbo molecular pump, a turbine generator, a machine tool, or the like.
However, in such a magnetic bearing, when a rotor shaft is rotated, due to a magnetic flux of electromagnets included in the magnetic bearing, an eddy current causes an iron loss in the rotor shaft, which may increase a temperature of the rotor shaft. This causes a problem such as a reduction in a tolerable flow rate of a vacuum pump including the magnetic bearing. At this time, when the vacuum pump is disposed perpendicular, by reducing currents (excitation currents) in the electromagnets included in the magnetic bearing, a rise in the temperature of the rotor shaft can be suppressed. However, in the case where the currents in the electromagnets are reduced, even though the vacuum pump is disposed perpendicular and stably rotated, when the vacuum pump is disposed horizontal again, a weight of the rotor shaft is applied in a radial direction to the electromagnets. Consequently, the rigidity (floating repulsive force) of the magnetic bearing decreases and the rotor shaft may not stably rotate any longer. Conversely, in the case where the vacuum pump is disposed horizontal and the rotor shaft is stably rotated, when the vacuum pump is disposed perpendicular again, the rigidity of the magnetic bearing (repulsive force toward a shaft center) excessively increases so that oscillation or vibration due to a shaft center shift or the like is more likely to occur.
That is, in accordance with the state of installment of a semiconductor manufacturing device or the like, a vacuum pump can freely be mounted in any mounting posture such as in a vertical direction, an inclined direction, a horizontal direction, or an inverted direction. At this time, to allow optimal currents to flow to the electromagnets of the magnetic bearing in accordance with a mounting posture of the vacuum pump, the currents flowing from the controller to the electromagnets should optimally be controlled. Accordingly, a technique for a magnetic bearing device is disclosed which selectively switches between control constants for a current compensation circuit so as to allow constantly optimal currents to flow in accordance with a direction in which the vacuum pump (i.e., rotor shaft of the magnetic bearing) is disposed. At this time, as the control constants to be selectively switched, constants determined in advance by experiment or the like in accordance with the direction in which the vacuum pump is disposed are used (see, e.g., Japanese Patent Application Publication No. H9-42290).